In many applications, it is necessary to accurately determine the wavelength(s) of light incident on a suitable detector. A widely used type of detector includes an etalon to filter specific frequencies of light. An etalon is a type of interference filter in which the intensity of transmitted light is dependent on its wavelength. In a conventional design, an etalon is comprised of two partially reflective parallel surfaces a distance d apart and separated by a material with an index of refraction r. When collimated light having a wavelength .lambda. is passed through the etalon, some of the light is reflected from the surfaces. The multiply reflected light beams interfere, either constructively or destructively, with each other, and thus alter the overall intensity of the light which passes through the etalon. Maximum transmission occurs when twice the distance between the reflective surfaces is an integral number of wavelengths .lambda. in the etalon. In other words, 2d*r/.lambda.=x, where x is an integer.
Often, it is desirable to provide a sensor which is sensitive to, and can discriminate among, several different frequencies of incident light at the same time. Such a sensor is particularly useful for spectrographic analysis. Although several discrete etalons can be utilized for this purpose, in some implementations, a stepped etalon is used instead. In this arrangement, one or both active surfaces of the etalon are stepped so that each step on the etalon provides a region of different thickness. By adjusting the thicknesses appropriately, each step can be configured to pass different frequencies of light. Stepped spectrographic etalon arrangements of this type are shown in U.S. Pat. No. 4,822,998 to Yokota et al. and U.S. Pat. No. 5,144,498 to Vincent.
A newly developed application requires a specifically configured stepped etalon to tune the output frequency of a laser. For fiber optic communications in particular, accurate tuning of the communication lasers is necessary to permit adjacent transmission channels to be closely spaced, often at wavelengths differing by only 0.4 nanometers or less. For such closely spaced channels, a laser's wavelength must be tuned to the assigned channel with an accuracy of +/-0.1 nanometers or less. Although only a single wavelength of light needs to be detected to tune such a laser, at these high accuracies, thermal variations in the thickness of an etalon and slight variations in the angle of applied light from normal to the etalon surface can shift the light transfer function an unacceptable degree.
According to the new application, the nominal thickness of the etalon can be chosen so that the periodicity of the etalon filter roughly matches the periodicity of a data communication channel spacing, i.e., 1500.12, 1550.52 nm for a system with a channel separation of substantially 0.4 nm. Two or more steps are formed on one side of the etalon. The step size is selected to be a fraction of the channel separation, on the order of 0.1 nm, and is substantially optimized so that a peak or trough in the transmission curve in the region of one step overlaps a steep portion of the transmission curve for one or more other steps. In this manner, as thermal changes in the etalon shift the transmission curve for one step beyond the desired range, the curve for a second step is shifted into the desired frequency. By selecting a particular step according to a measured temperature and etalon calibration information, and measuring the intensity of laser light transmitted through the selected step of the etalon, a feedback signal is provided which can be used to adjust the output wavelength of the laser. Similarly, different steps can be selected to compensate for tolerance errors in the angle of light incident to the etalon.
With reference to FIGS. 1a and 1b, in both views of stepped etalon configurations, the stepped etalon 10 has partially reflective coatings 11a, 11b and is positioned adjacent an appropriately configured array of photodetectors 14a, 14b, where each detector is aligned with a corresponding etalon step land 12a, 12b. When a beam of light 16 is directed onto the etalon 10, the intensity of the output signal attributed to each detector 14a, 14b indicates the intensity of light passing through the etalon in the region of the corresponding step, therefore providing a measure of the intensity of incident light, with the particular frequencies determined by the thickness of the etalon in that region.
A significant drawback to a conventional stepped etalon is the interference caused by the abrupt transition between the lands of adjacent steps. When no step is present, the intensity within a collimated light beam transmitted through an etalon has the same intensity pattern as the incident beam, typically gaussian-like as shown in FIG. 1c. However, when an abrupt step is present, the incident and resonant light is diffracted by the step wall 18, producing interference within the transmitted beam along the z-axis (perpendicular to the step edge). The resulting fringe pattern is illustrated in FIG. 1d. The result of the diffraction is that in the vicinity of the step, there is substantial angular dispersion of the light which reduces the quality of the transmission function resulting in reduced signal amplitude, broadened peaks, as well as reduced ability to differentiate small changes in the frequency of the input light.
Such a reduction in wavelength discrimination is illustrated in FIG. 1e for a two step etalon. Curves A1-A5 are measured on a first step A and curves B1-B5 are measured on a second step B. Curves A1 and B1 represent positions distant from the step wall. The remaining curves A2-A5 and B2-B5 are measurements made at locations progressively closer to the step wall. The input signal is provided by a temperature tuned laser and therefore increases in temperature represent increases in input signal wavelength. As indicated, the peaks and troughs for curves close to the step transition are lower and less defined than those measured far from the step transition, indicating that near the step transition, it is harder to discriminate between wavelengths that are close to each other.
The effect of the interference and overall reduction in etalon quality associated with abrupt steps creates a "dead spot" behind and near the step edge in which accurate intensity readings are compromised. Thus, there are portions of the etalon where a detector cannot be placed due to the reduced quality of the transmitted beam.
For example, experiments using an etalon with a thickness of approximately 2 mm and a step height of approximately 0.2 um reveal a "dead spot" approximately 600 to 800 um wide directly behind the step. Since input beam widths of between 0.5 to 5.0 mm are common, a significant portion of the transmitted beam will not have high quality etalon transmission characteristics and thus will not be suitable for detection. This reduces the available optical power for measurement and lowers the power-per-detector. Since a minimum signal-to-noise ratio is required for reliable measurements, decreasing the power-per-detector thus can decrease the accuracy of the detector and the stability of equipment which is adjusted according to the etalon measurements. The interference also limits the number of possible steps which can be placed on an etalon of a given size.
Although the size of the etalon can be increased to provide more area within each step land which is distant from the edge, this is often an undesirable solution. First, the detector array is commonly formed on an integrated circuit which may not be as easily increased in size without a relatively large increase in production cost. Second, the width of the input light beam itself may not be variable and increasing the etalon width will introduce the additional problems of directing the beam to the desired portion of the etalon.